1. Field of the Invention
The invention relates in general to electrical inductive apparatus and in particular to an annunciator for multiple protective and/or monitoring devices for such apparatus.
2. Description of the Prior Art
When a transformer or other electrical apparatus is subjected to an internal fault or mechanical failure, one or more protective devices respond by sending a signal from the internal sensing means to a central point. The function of the annunciator is to monitor a multiple of fault indications at the central point, at which point the annunciator "alarms" an abnormal condition, usually by audio means such as an alarm or siren. The annunciator also distinguishes the abnormal condition, usually by visual means such as a lamp or mechanical target.
While the malfunction is being repaired, it is normal practice for an operator to silence the audio alarm, usually by opening a set of contacts in the alarm circuit. This practice is dangerous, as a subsequent malfunction will not be detectable by audio means. Also, the alarm silencing procedure requires the operator to reset the alarm circuit after the malfunction has been repaired. If the operator should forget to reset the alarm circuit, there would again be no audible signal to alert the operator of a subsequent fault or abnormal condition.
An annunciator which has the capability of silencing the alarm without defeating the ability of the annunciator circuit to alarm a subsequent fault condition is usually found in an annunciator employing solid-state circuitry. However, annunciators utilizing solid-state components have characteristics that are not suitable for the environment of electrical power apparatus, particularly the temperatures and transients that power and distribution transformers are subjected to. Another problem with solid-state devices is that when failure does occur due to an over-voltage or current transient, the device usually must be replaced, as it is not susceptible to on-site repair. A still further problem with annunciators employing solid-state circuitry is that the device cannot withstand standard tests that are applied to transformers as part of the assembly process. Power transformer auxiliary control wiring is normally tested for insulation integrity by applying approximately 1500 volts for one minute between components and ground. Solid-state devices as supplied in annunciators rarely permit application of test voltages above 600 volts. During the insulation tests the transformer annunciators utilizing solid-state components must be disconnected and either tested separately at a lower voltage or not tested at all.
Circuits employing electro-mechanical relay components would be suitable for the type of electrical power apparatus under discussion. One such annunciator employs a series of relays with "passing contacts" which are dependent upon gravity for their operation, to satisfy the alarm reset condition. However, the relay components are susceptible to malfunction due to the normal vibrations of a transformer during its operation. Care must also be taken with this particular annunciator in the mounting of it on the transformer tank wall or other desired mounting areas such that all of the passing contacts are in a position to utilize the pull of gravity for their operation. Another liability of this type of annunciator is that the multiple indication circuits feed directly to the alarm relay circuit. During simultaneous operation of several protective devices, an alarm signal may be lost due to undercapacity of the alarm circuit to handle multiple alarm signals.
It would be desirable to have an annunciator that utilizes standard electro-mechanical relay components and provides the capability of resetting the alarm without defeating the ability to respond to a subsequent malfunction. In addition it would be desirable for the annunciator to respond to simultaneous signals from multiple protective devices without losing any individual signal.